2,5-Dioxopyrrolidin-1-yl propanoate is a preformed active ester of propanoic acid with N-hydroxysuccinimide (NHS). It enables efficient and selective acylation of primary amines to form amide bonds under mild, aqueous conditions without the need for additional activation. Widely used in bioconjugation for labeling and modifying biomolecules such as proteins, antibodies, and peptides with the propanoyl group. Its stability in physiological pH and temperature reduces side reactions, making it ideal for solid-phase peptide synthesis and the preparation of pharmaceutical intermediates.